1. Field of the Invention
The present invention relates to an image display including pixel circuits for driving light-emitting elements provided on each pixel basis by current. More specifically, the invention relates to a so-called active-matrix image display in which pixel circuits are arranged in a matrix (in rows and columns) and, in particular, the amounts of currents applied to light-emitting elements such as organic EL elements are controlled by insulated-gate field effect transistors provided in the pixel circuits.
2. Description of the Related Art
In an image display, e.g., in a liquid crystal display, a large number of liquid crystal pixels are arranged in a matrix, and the transmittance intensity or reflection intensity of incident light is controlled on each pixel basis in accordance with information on an image to be displayed, to thereby display the image. This pixel-by-pixel control is implemented also in an organic EL display employing organic EL elements for its pixels. The organic EL element however is a self-luminous element unlike the liquid crystal pixel. Therefore, the organic EL display has the following advantages over the liquid crystal display: higher image visibility, no necessity for a backlight, and higher response speed. Furthermore, the organic EL display is a current-control display, which can control the luminance level (grayscale) of each light-emitting element based on the current flowing through the light-emitting element, and hence is greatly different from the liquid crystal display, which is a voltage-control display.
The kinds of drive systems for the organic EL display include a simple-matrix system and an active-matrix system similarly to the liquid crystal display. The simple-matrix system has a simpler configuration but involves problems such as a difficulty in the realization of a large-size and high-definition display. Therefore, currently, the active-matrix displays are being developed more actively. In the active-matrix system, a current that flows through a light-emitting element in each pixel circuit is controlled by active elements (typically thin film transistors (TFTs)) provided in the pixel circuit. An example of the pixel circuit is disclosed in Japanese Patent Laid-open No. Hei 8-234683.
FIG. 1 is a circuit diagram showing a typical example of an existing pixel circuit. As shown in the drawing, the existing pixel circuit is disposed at the intersection between a row scan line WS that supplies a control signal and a column signal line SL that supplies a video signal. The pixel circuit includes at least a sampling transistor T1, a pixel capacitor Cs serving as a capacitive part, a drive transistor Td, and a light-emitting element OLED. The sampling transistor T1 conducts in response to the control signal (selection pulse) supplied from the scan line WS to thereby sample the video signal supplied from the signal line SL. The pixel capacitor Cs holds an input voltage dependent upon the sampled video signal. The drive transistor Td is connected to a power supply line Vcc and supplies an output current to the light-emitting element OLED depending on the input voltage held by the pixel capacitor Cs. The light-emitting element OLED is a two-terminal element (diode-type element). The anode thereof is connected to the drive transistor Td, while the cathode thereof is connected to a ground line GND. The light-emitting element OLED emits light with a luminance dependent upon the video signal due to the output current (drain current) supplied from the drive transistor Td. In general, the output current (drain current) has a dependency on the carrier mobility in the channel region of the drive transistor Td and the threshold voltage of the drive transistor Td.
The drive transistor Td receives by its gate the input voltage held by the pixel capacitor (capacitive part) Cs and conducts the output current between its source and drain, to thereby apply the current to the light-emitting element OLED. The light-emitting element OLED is formed of e.g. an organic EL device, and the light emission luminance thereof is in proportion to the amount of the current applied thereto. The amount of the output current supplied from the drive transistor Td is controlled by the gate voltage, i.e., the input voltage written to the pixel capacitor Cs. The existing pixel circuit changes the input voltage applied to the gate of the drive transistor Td depending on the input video signal, to thereby control the amount of the current supplied to the light-emitting element OLED.
The operating characteristic of the drive transistor is expressed by Equation 1.Ids=(½)ρ(W/L)Cox(Vgs−Vth)2  Equation 1
In Equation 1, Ids denotes the drain current flowing between the source and drain. This current is the output current supplied to the light-emitting element in the pixel circuit. Vgs denotes the gate voltage applied to the gate with respect to the potential at the source. The gate voltage is the above-described input voltage in the pixel circuit. Vth denotes the threshold voltage of the transistor. μ denotes the mobility in the semiconductor thin film serving as the channel of the transistor. In addition, W, L and Cox denote the channel width, channel length and gate capacitance, respectively. As is apparent from Equation 1 as a transistor characteristic equation, when a thin-film transistor operates in its saturation region, the transistor is turned on to conduct the drain current Ids if the gate voltage Vgs is higher than the threshold voltage Vth. In principle, a constant gate voltage Vgs invariably supplies the same drain current Ids to the light-emitting element as shown by Equation 1. Therefore, supplying video signals at the same level to all the pixels in a screen will allow all the pixels to emit light with the same luminance, and thus will offer uniformity of the screen.
However, actual thin film transistors (TFTs) formed of a semiconductor thin film such as a poly-silicon film involve variation in the device characteristics. In particular, the threshold voltage Vth is not constant but varies from pixel to pixel. As is apparent from Equation 1, even if the gate voltage Vgs is constant, variation in the threshold voltage Vth of the drive transistors leads to variation in the drain current Ids. Thus, the luminance varies from pixel to pixel, which spoils uniformity of the screen.
To address this, there has been developed a pixel circuit provided with a function to cancel the variation in the threshold voltage of drive transistors. This pixel circuit is disclosed in e.g. Japanese Patent Laid-open No. 2005-345722.
The pixel circuit provided with the function to cancel variation in the threshold voltage Vth can improve uniformity of a screen and can address luminance variation due to changes of the threshold voltage over time. However, to provide the pixel circuit with the threshold voltage cancel function, there is a need to add at least three transistors to the sampling transistor and the drive transistor. In addition, these added transistors need to be line-sequentially scanned at timings different from the timings for the sampling transistors. Consequently, unlike the simple pixel circuit shown in FIG. 1, at least four scan lines are required for pixels on one row, and correspondingly scanners for line-sequentially scanning the respective scan lines at different timings are required. That is, compared with in the simple pixel circuit shown in FIG. 1, the number of the scanners is increased by three for the line-sequential scanning of the pixels provided with the threshold voltage cancel function. When the pixel circuits are formed by an amorphous-silicon TFT process, the scanners are formed of external components in general. Therefore, the increase in the number of the scanners directly leads to increase in the manufacturing costs. When the pixel circuits are formed by a low-temperature poly-silicon TFT process, it is possible to form the scanners by use of poly-silicon TFTs simultaneously. However, the increase in the number of the scanners contributes to a yield decrease and requires the space for arrangement of the scanners on the substrate. As a result, the manufacturing costs increase.